The present invention relates to a method for regenerating a spent solvent discharged from a solvent extraction process in a nuclear fuel cycle, such as in reprocessing facilities for spent nuclear fuel or in nuclear fuel manufacturing facilities.
A phospate, such as tributyl phosphate (TBP), diluted with a higher hydrocarbon such as n-dodecane (hereinafter referred to simply as "dodecane") or kerosine is widely used as a solvent in a solvent extraction process in the reprocessing of a spent nuclear fuel or in wet scrap recovery process in nuclear fuel manufacturing facilities.
The spent solvent which was used in the solvent extraction step contains degradation products such as dibutyl phosphate (DBP) and monobutyl phosphate (MBP) formed by the decomposition of a part of TBP by an acid, heat or radiation. Since such degradation products exert a bad influence on phase separation or extraction efficiency when the spent solvent is recycled for reuse, the degradation products are removed by alkali washing with an aqueous solution of sodium hydroxide or sodium carbonate. A radioactive waste containing the degradation products such as DBP thus removed is mixed with a vitrification additive or bituminization additive to form a vitrifed or bituminized solid.
Further, in order to separate TBP, DBP, dodecane, etc. in the spent solvent, there have been used methods such as vacuum freeze-drying and low-temperature vacuum distillation wherein a difference in the boiling point among them is utilized.
However, the method wherein the degradation products are removed by the alkali washing has a defect in that a large amount of the vitrification additive or bituminization additive is necessitated in the vitrification or bituminization of the radioactive waste for stabilizing sodium which has been introduced during the alkali washing, to thereby increase the amount of the waste. Under these circumstances, the development of a method for regenerating the spent solvent which enables the degradation products such as DBP to be removed without using sodium is demanded.
The vacuum freeze-drying and low-temperature vacuum distillation methods have a defect in that a large amount of energy is required to achieve the requisite low temperature and the treatment capacity is small.